Lithium ion cells are the newest of the commercially available battery cells. They are higher in energy density per unit weight than silver oxide, mercury or alkaline dry cells and are one of the best performers at very high and low temperatures. Lithium cells also have a longer shelf life than many other batteries and thus have been the choice for many in battery design. A typical laminar lithium ion cell battery includes an anode of lithium metal or of a lithium insertion compound; a separator structure or electrolyte film layer; a cathode including electrochemical active material, typically a chalcogenide of a transition metal; and an electrolyte prepared from an inorganic lithium salt dissolved in an organic solvent. These batteries are well-known in the art and examples of such are set forth in U.S. Pat. Nos. 4,997,732; 5,456,000.
During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode, where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the lithium anode.
Numerous technical problems are encountered, however, in the construction of solid state electrochemical cells, particularly in the establishment of electrode/separator and electrode/current collector interfaces. Failure to establish satisfactory interfaces may manifest itself in high cell impedance and poor discharge performance. The low electrode impedances required for good battery performance can be enhanced by bringing the planar surfaces of the various layers into intimate contact with their adjoining layers. The prior art method to secure adequate contact is to seal the battery under vacuum, so that compression of the battery cell is achieved by atmospheric pressure.
U.S. Pat. No. 4,997,732 to Austin et al., describes a known method of compressing the stacks of a battery cell. A layer of insulating envelope material is sealed around the battery cell under a vacuum. This enables the envelope material to adhere to the laminar cell to prevent the cell from moving within the sealed enclosure. The vacuum sealed cell envelope helps prevent delamination of the component layers. The vacuum inside the cell layers results in a pressure differential across the envelope so that the atmospheric pressure brings the electrodes into intimate contact with the electrolyte.
Problems with this solution occur if the vacuum inside the cell envelope is reduced. If the pressure differential across the envelope is reduced substantially, the pressure differential applied to the cell components will no longer be adequate to produce the compression force needed to insure component contact. Vacuum reduction is not uncommon and may occur for a number of reasons. For example, vacuum reduction may occur from gas formation due to impurities inside the cell envelope or from leakage of the envelope due to puncture.
In devices where the vacuum is successfully maintained, other problems may still occur. The vacuum inside the sealed packaging producing the pressure differential also acts as a driving force to push outside air into the cell when the envelope is punctured. Because metallic lithium is reactive with all environmentally present gases and vapors other than noble gases, it is particularly susceptible to contamination by, or reactivity with, environmental materials.
U.S. Pat. No. 5,456,000, which is incorporated by reference in its entirety, discloses the formation of electrolytic cell electrodes and separator elements. The electrodes and separator elements use a combination of a poly(vinylidene fluoride) copolymer matrix and a compatible organic solvent plasticizer to provide battery component layers, each in the form of a flexible, self-supporting film.
An electrolytic cell precursor, such as a rechargeable battery cell precursor, is constructed by means of the lamination of electrode and separator cell elements which are individually prepared. Each of the electrodes and the separator is formed individually, for example by coating, extrusion, or otherwise, from compositions including the copolymer materials and a plasticizer. The materials are then laminated as shown in FIG. 1.
In the construction of a lithium-ion battery, for example, a copper grid may comprise the anodic current collector 110. An anode (negative electrode) membrane 112 is formed by providing an anodic material dispersed in a copolymer matrix. For example, the anodic material and the copolymer matrix can be provided in a carrier liquid, which is then volatilized to provide the dried anode membrane 112. The anode membrane 112 is positioned adjacent the anodic current collector 110.
A separator membrane 114 is formed as a sheet of a copolymeric matrix solution and a plasticizer solvent. The separator membrane 114 is placed adjacent the anode membrane 112.
A cathode (positive electrode) membrane 116 is similarly formed by providing a cathodic material dispersed in a copolymer matrix. For example, the cathodic material and the copolymer matrix can be provided in a carrier liquid, which is then volatilized to provide the dried cathode membrane 116. The cathode membrane 116 is then overlaid upon the separator layer 114, and a cathodic current collector 118 is laid upon the cathode membrane.
The assembly is then heated under pressure to provide heat-fused bonding between the plasticized copolymer matrix components and the collector grids. A unitary flexible battery precursor structure is thus produced. Generally, the plasticizer is removed using a solvent such as ether or hexane. This produces a "dry" battery precursor substantially free of plasticizer and which does not include any electrolytic solvent or salt. An electrolytic solvent and electrolyte salt solution is imbibed into the "dry" battery copolymer membrane structure to yield a functional battery system.
There is a need to protect lithium ion battery components from environmental materials, and to protect the environment from both the battery materials and from byproducts of battery operation. Batteries of the prior art have been sealed under vacuum and/or placed in protective housings to provide a barrier between the battery and the environment. The effectiveness of a vacuum seal can be compromised by either variations in the ambient pressure, or a buildup of gaseous or liquid byproducts within the package.
Physical contact between individual layers of the battery (for example, the anode/separator interface, the cathode/separator interface, and the electrode/current collector interfaces) may be incomplete, or gapped, leading to reduced battery performance.